RF Shielding
Radiofrequency (RF) shielding is the practice of blocking radiofrequency electromagnetic signals that cause radio frequency interference (RFI). RFI can disrupt the electrical circuits of a device from working normally...
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This article provides a complete guide to EMI and EMIF shielding.
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EMI shielding is a technique of creating a barrier that prevents leakage of strong electromagnetic fields that can interfere with sensitive devices and signals. They can be installed to isolate the electromagnetic field source or as an enclosure of the device that needs protection. Electromagnetic interference, or radio frequency interference (RFI), is a problem for most electronics since it can decrease the performance of the circuit or even cause it to fail. Electronics deal with small voltages and currents that an electromagnetic field can easily disrupt.
Electromagnetic interference (EMI) is the coupling of signals from one system to another. There are three components to creating an EMI: the source, path, and receiver. The two systems are the source and the receiver. The source is generally the external circuit or phenomenon that creates the disturbance, which can be naturally occurring (lightning, auroras, cosmic microwave background, and solar flares) or artificial (cellular networks, AM/FM radio waves, power transmission lines, measurement and control devices). The receiver, or the victim, is the sensitive signal or device in which its output signal is distorted by the interference. The path is where signal coupling occurs, which can be through four modes.
EMI can be classified according to the duration of the interference. Types of interference are continuous and pulse interference. A continuous interference is a type of EMI where the source continuously emits the unwanted signal. Continuous interference is generally characterized as low energy and low frequency. Continuous interference includes radio frequencies, electromagnetic field leaks from industrial equipment, power transmission lines, etc. On the other hand, pulse, intermittent, or transient interference is an EMI that occurs only in a short duration. The definition of its duration varies from each application, but the usual is less than a period of one AC cycle (1/60 Hz = 16.67 milliseconds). Pulse interference is characterized by high bursts of energy, which can be repetitive or random events. Repetitive is usually artificial, making it predictable in terms of amplitude and duration. Random events can be artificial and naturally occurring, such as lightning strikes, power surges, electrostatic discharge, and so forth.
Another classification of EMI is by the length of the wave bandwidth of the disturbance, which can be narrowband or broadband. The definition of the two depends on the bandwidth of the signal on the receiver, termed the resolution bandwidth. A narrowband disturbance has a bandwidth of less than or equal to the receiver, while a broadband disturbance has greater bandwidth.
Every day, new innovations are flooding the commercial market with new conveniences that are designed to make life easier. This wide array of electronics has the potential to damage and interfere with each other unless they are shielded properly. EMI shielding aims to isolate the energy of a device so that it does not affect what is around it and blocks external energy from attacking. Every new electronic device emits some amount of electromagnetic energy and has zero resistance to EMI.
With an understanding of the nature of EMI, it is clear that it can affect its surroundings in various ways. It can affect electronics that are in contact (conducted EMI), in close proximity without contact (capacitive and magnetic EMI), and even over large distances (radiated EMI). Along with the progress of the information age, the increased usage of electronics for data processing and communication creates considerable pollution to the electromagnetic wave spectrum, on top of the other disturbances caused by electrical transmission and distribution systems and natural phenomena such as lightning strikes and solar flares. Below are some effects of EMI.
International organizations developed electromagnetic compatibility (EMC) standards to address these problems. EMC is the property or characteristic of equipment to operate correctly in an electromagnetic environment without generating or transmitting electromagnetic energy to other equipment. International EMC standards are stipulated by the International Special Committee on Radio Interference (CISPR), a part of the International Electrotechnical Commission (IEC), and the International Organization for Standardization (ISO). In the United States, EMC standards are managed by the Federal Communications Commission (FCC).
There are many machines available to produce EMI (Electromagnetic Interference) shielding, which are important in today's society as they enable the manufacturing of components and devices that provide protection against electromagnetic interference, ensuring reliable and safe operation of electronic systems in industries such as telecommunications, aerospace, medical devices, and automotive. Below, we study many leading brands of machines available in the United States and Canada used to create EMI shielding materials, along with specific models and their unique abilities, features, or components:
Features: Chomerics offers manufacturing equipment for producing EMI shielding materials, such as conductive gaskets, films, and coatings. Their equipment includes various machinery, such as extruders, calenders, coaters, and curing systems and allows for precise control over material thickness, conductivity, and other properties. It supports different manufacturing processes, including extrusion, coating, and lamination. Chomerics' equipment is designed for high-volume production and customization of EMI shielding materials.
Features: Schaffner Group provides manufacturing systems for producing EMI shielding solutions, including filters, contact strips, and ferrites. Their systems include machinery for metal stamping, injection molding, and assembly. These systems offer precise tooling and production capabilities for high-quality EMI shielding components and incorporate automation features for efficient and cost-effective manufacturing processes. Schaffner Group's manufacturing systems are designed to meet industry standards and customer-specific requirements.
Features: Integrated Microwave Corporation specializes in equipment for producing EMI/RFI shielding materials, such as conductive foams and absorbers. Their production equipment includes laminating machines, die-cutting systems, and custom tooling solutions. This equipment enables precise fabrication and customization of EMI shielding materials to meet specific requirements and supports various manufacturing processes, including lamination, cutting, and shaping. IMC's production equipment ensures high-quality EMI/RFI shielding materials with consistent performance.
Features: JEMIC Shielding Technology offers many machines for manufacturing EMI shielding materials, including conductive fabrics, tapes, and gaskets. Their machines include coating systems, cutting machines, and sewing machines. Their machines allow for precise coating, cutting, and assembly of EMI shielding materials. They offer flexibility in material selection, thickness, and configuration and are designed for efficient production and customization of EMI shielding products.
Features: Leader Tech Inc. provides equipment for manufacturing EMI shielding materials, such as conductive elastomers, vents, and thermal pads. Their equipment includes mixing and dispensing systems, molding presses, and curing ovens. Their equipment enables precise material formulation, molding, and curing processes for consistent EMI shielding performance. It also supports customization options, including material properties, shapes, and sizes. Leader Tech's manufacturing equipment ensures high-quality EMI shielding materials with reliable performance.
Please note that specific model availability and features may vary, so it is advisable to contact the manufacturers or their authorized distributors for the most up-to-date information on the models that suit your requirements.
EMI shielding is one of the methods of achieving EMC aside from grounding, filtering, and bonding techniques. This involves creating enclosures with the appropriate material, structure, and form to alter the path of most undesired electromagnetic waves coming into or out of the equipment. The path is altered by absorbing or reflecting the electromagnetic wave through conductive or ferromagnetic materials.
An electromagnetic wave consists of an electric component and a magnetic component. Both of them travel at the same frequency and are perpendicular to each other. A conductive material blocks the electric components, while a material with high magnetic permeability blocks the magnetic components. Since a component of an electromagnetic wave cannot exist without the other, it is enough to protect one component. When it comes to EMI shielding, there are different mechanisms involved to filter out each. Enumerated below are the three mechanisms of EMI shielding.
The main mechanism of EMI shielding is reflection, which attenuates the electric component of the EMI. In order to achieve EMI reflection, the material must have mobile charge carriers. This means the material used for shielding must be conductive. The incoming electromagnetic wave interacts with the mobile charge carriers present in the conductive shield. This interaction causes the charges to flow and redistribute along the conductor creating an opposing electromagnetic field. The electromagnetic field generated by the redistribution of charges cancels out the external magnetic field. In this mechanism, the higher the conductivity of the material, the better the shielding characteristics.
The problem with this mechanism is that a discontinuity on the enclosure that is larger than the wavelength of the external electromagnetic field will defeat its shielding properties. Thus, the sizes of holes and openings in the enclosure design are minimized. However, this is not possible for higher electromagnetic wave frequencies. The only way to counter this in high-frequency EMI is through the use of filtering devices.
Another problem is the skin effect, which is seen in AC circuits. When AC flows through the conductor, the charges accumulate at the surface or the top-most layers of the conductor, increasing the current density in that area. The inner section is used less, which lowers the conductivity and ultimately, the performance of the shield. This effect is highly evident in high-frequency electromagnetic waves. A solution for this is to increase the conductor's surface area, thereby increasing the effective conducting cross-section. Another solution is by electroplating the surface with a highly conductive material at the surface such as silver.
This is the secondary mechanism of EMI shielding which acts on the magnetic component of the EMI. To achieve EMI absorption, the material must have electric and magnetic dipoles. These are materials with high dielectric constant and high magnetic permeability. In the presence of an external magnetic field, the magnetic field lines are cut since they tend to travel through the material. An enclosure with this property absorbs the magnetic ane electric field lines by creating a pathway within itself. However, a problem in using these materials is that they do not have high conductivity. Thus, they are less efficient in protecting from the electric component of the electromagnetic wave.
Part of the absorption mechanism is weakening of the incoming electromagnetic wave through eddy currents. This is observed when the electromagnetic wave is oscillating at a high frequency, which induces currents within the conductor. The eddy currents create their magnetic field that opposes the external magnetic field. Materials with high electric conductivity create stronger eddy currents.
This is another mechanism observed in composite materials with large interfacial areas or surfaces with porous structures. Shielding is achieved by having multiple reflecting boundaries for reflecting the electromagnetic wave. This results in the scattering of electromagnetic waves.
The previous chapter discussed the mechanism of EMI shielding and the desired properties of materials necessary to achieve it. Two main properties achieve EMI shielding: electrical conductivity and magnetic permeability. Below are some of the materials that demonstrate these properties.
Metals: Metals are the first choice for cheap and simple EMI shielding. Their properties such as electrical conductivity, magnetic permeability, strength, and ductility make them suitable for shielding structural materials. Silver has the best electrical conductivity with good corrosion resistance, making it the most efficient electric field attenuator. The downside of using silver is its relatively high cost compared to other metals, which is why it is used as an alloying component or surface coating through electroplating. Balancing the cost and shielding efficiency, copper and aluminum are the most widely used metals for EMI shielding. The electrical conductivity of copper is almost the same as silver, while that of aluminum is 40% less.
Carbon steel alloys such as mild carbon steel and ferritic stainless steel, and iron-nickel alloys such as Mu-metal, Permalloy, and Supermalloy are common materials used for magnetic shielding. The most popular is Mu-metal which has a relative permeability of 100,000 at 1kHz.
Carbon Allotropes: Carbon allotropes are forms of carbon, such as exfoliated graphite, graphene, carbon fibers, and carbon nanotubes. They are used as filler materials for EMI shield composites. They are effective filler materials due to their intrinsic strength and conductivity. They mainly operate through the multiple reflection mechanism of shielding. Exfoliated graphite is widely used as EMI shielding gaskets due to its flexibility and ability to flow on the surface irregularities of the sealing surfaces. They have a highly porous structure that promotes EMI absorption. Graphene, carbon fibers, and carbon nanotubes are used as filler materials due to their high aspect ratio. They are commonly embedded in polymers, ceramics, cement, and metals to create rigid structures. For high-frequency shielding applications, graphene and carbon nanotubes are mostly used because the dimension of these materials is lesser than the skin depth. This makes them better conductors than metals in the GHz range.
Intrinsically Conducting Polymers (ICPs): These are special polymers that can conduct electricity within themselves without the need for additional conducting materials. They are desired because of their light weight and processability. ICPs can conduct electricity between atoms due to the conjugated bonds (alternating single and double bonds). This enables the delocalization of π-electrons (loose electrons), which act as mobile charges. The electric conducting property of ICPs can be modified through doping or de-doping. Popular ICPs are polyaniline (PANI) and polypyrrole (PPy). The use of ICPs is still under development since several problems exist concerning their mechanical and chemical stability. They are more extensively used as components to composites containing metal nanoparticles and carbon filaments.
Foam: The type of foam used for EMI shielding is carbon foam, which is lightweight, high temperature tolerant, and has adjustable thermal and electrical properties. The two types of carbon foam are graphitic and non-graphitic. Non-graphitic foams are stronger, can be used as a thermal insulator, and cost less.
Graphitic foam is made from petroleum, coat tar, or synthetic pitch and is more expensive to produce. Due to the expense of production, graphitic foam is in limited supply but is widely used for EMI shields. Used as an EMI shield, it is capable of shielding radio frequencies from 100 Mhz up to 20 GHz without any metal component.
Foam is an ideal material for EMI shielding due to its flexibility and adaptability. These characteristics have made it a popular choice for EMI shields. Additionally, foam lasts longer in extreme and harsh conditions, weighs very little, and does not rust or corrode. Foam is the most environmentally friendly of the many materials used to make EMI shields.
Fabric: Although EMI fabric may seem to be traditional fabric, it is used like textile fabric in EMI protection and has physical properties like conventional fabric. The substrate of EMI fabric is nylon or polyester that has been interwoven with metal. The flexibility of EMI fabric makes it possible to engineer it so that it can be effective in a wide variety of conditions and applications.
EMI fabric is not the strongest form of EMI protection, but it is ideal for conditions where a moderate amount of protection is required. Since EMI fabric has a metal content, it tends to get surface corrosion in certain conditions.
EMI shielding can be considered the most cost-effective method of EMC compatibility since it decreases the use of intra-equipment devices to manage undesired signals. Achieving EMC through shielding depends on two factors: the structural or form design and the materials used. A simple EMI shield design is a Faraday cage made of conductive materials such as copper. The EMI shield design depends on the characteristics of the electromagnetic environment within which the equipment must reliably function. Below are some design considerations in constructing an EMI shield.
For uneven surfaces, it is best to use an EMI shielding gasket. EMI shielding gaskets are commonly used for enclosures with removable panels, drawers, and covers. The gaskets fill in the gaps to provide continuous electrical contact between surfaces. The following properties must be considered in selecting an EMI shielding gasket:
Shielded cables are used for signal lines penetrating shielded enclosures. These cables are grounded to the outer shield of the enclosure.
EMI shielding components have varying forms depending on the application. Below are the common EMI shields seen in the market.
Solid Enclosures: Typically, metallic cases have sufficient rigidity to contain and support the device. Thus, it serves two functions. First, it acts as structural support or frame to the unit. Second, it prevents electromagnetic waves from getting into or out of the system. The enclosure is grounded so that any stray current will be diverted to ground, which minimizes the risk of electric shock.
Wire Mesh and Screens: These are shielding materials with penetrations or discontinuities less than the expected wavelength of the EMI. They function similarly to solid enclosures, but with the added benefit of allowing ventilation, especially for electronics that generate heat. Moreover, using wire mesh creates a translucent characteristic to an enclosure, which is useful for see-through enclosures and displays. Since the discontinuities need to be very small, they are manufactured through high-resolution processes such as photochemical etching and printing.
Gaskets and O-rings: As mentioned before, the enclosures must be fully continuous without any penetrations so as not to defeat the purpose of EMI shielding. In reality, this is not the case since all enclosures must have access to the components inside. Thus, there is a continuity problem with the removable parts of the enclosure. EMI shielding gaskets and O-rings solve this problem by being able to absorb EMI, and at the same time, have the flexibility and elastic characteristics of ordinary sealing materials.
Cable Shielding: These are tapes or wire braids that cover the insulated conductor of a power or instrument cable. When used in a power cable, this prevents creating electromagnetic waves that can affect nearby electronic components. When used in instrument cabling, the main function is to prevent external electromagnetic waves from distorting the signal that flows through the cable. The shielding is grounded so that any leaking current or stored electricity through parasitic capacitance is diverted to ground.
Coatings: EMI shielding materials can also be coated through painting, spraying, dispensing, and electroplating. These are typically used in lightweight applications, such as mobile devices, where metallic shielding is impractical.
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